1. Field of the Invention
The present invention relates in general to devices for draining oil out of a valve spool cap and, more particularly, to a structural improvement in such devices for rapidly draining the oil out of the valve spool cap and thereby improving responsibility of the valve spool.
2. Description of the Prior Art
With reference to FIG. 1, there is shown a valve with a typical oil drain device. As shown in this drawing, the valve includes a spool cap 103 for receiving an end of a valve spool 102 extending to the outside of a valve body 101. In the spool cap 103, the valve spool 102 is biased by a spring 104 at a given spring force. The spool cap 103 is filled with oil.
In order to move the spool 102 in the direction of arrow "a" of FIG. 1, the oil in chamber "A" of the spool cap 103 should be rapidly drained. The device for draining the oil in the valve is formed by a gap which is selectively defined between the interior end wall of the cap 103 and a spring guide 105 when applying pilot pressure to the spring guide 105. The oil in chamber A is drained to a drain tank (not shown) through the gap and through a drain line B.
However, the above oil drain device has a problem that the desired rapid responsibility of the spool 102 in response to the pilot pressure can not be achieved due to both the weight of the spool 102 and the negative pressure in the line B. That is, the above oil drain device can not achieve rapid oil draining even though the oil should be rapidly drained out of the spool cap 103 in order for improving the responsibility of the spool 102. In this regard, the typical valve needs relatively large pilot pressure in order to move the spool.
Japanese Patent Laid-open Publication Nos. Sho. 62-2004, 62-24079 and 62-48080 disclose use of a so-called "drain core" in the valve for rapidly draining the oil out of the spool cap and thereby improving the responsibility of the spool. As shown in FIG. 2A, each of the prior art valves include a groove 203 which is formed on a spool 202 to form a given gap between the exterior wall of the spool 202 and the interior wall of a valve body 201. The drain core 204 for directly draining the oil to the drain tank is formed in the valve body 201 such that the drain core 204 is opposed to the groove 203. In the above prior art valves, when a spring guide 205 moves rightward in the drawing as a result of movement by the spool 202 in the direction of arrow "a", the oil in the spool cap 206 flows in the space defined between the end wall of the spring guide 205 and the valve body 201 in the spool cap 206 and flows in the groove 203 of the spool 202, and in turn is drained to the drain tank through the drain core 204 as shown in FIG. 2B. However, each of the prior art valves has a problem that each valve has a time delay in oil draining due to the fact that the intended drain function is not activated until the spool 202 moves by at least the distance "1" by shifting the portion of groove 203 into the spool cap 206 and thereby making the inside of the spool cap 206 communicate with the drain core 204 of the body 201. Due to the time delay, the prior art valves can not achieve the desired responsibility of the spools.